Most plastic sheetings are hydrophobic, thus facilitating water condensation on their surface in the form of discrete droplets. Also water droplets impinging on such films do not spread out on the surface but retain the droplet shape.
When used as cover for greenhouses condensation of water vapor and droplet formation takes place on the inner surface of such sheetings, i.e. facing the interior of the greenhouse. After growth to a certain size, such drops become detached from the surface and fall onto growing plants, causing frequently damage to sensitive crops, such as flowers and also to leaves of plants. The presence of fogging thus deposited on the surface of leaves constitutes a focus for fungal spore germination which cause grave damage, such as, for example, Botrytis cinerea. Furthermore, fogging on the plastic sheet surface reduces light entering the greenhouse, frequently by as much as about 50 percent, This reduces photosynthesis during some hours, until heating clears up the droplets from the film surface. There have been tried sprays applied to such plastic sheeting, in order to reduce droplet formation on the film surface. Such sprays retain their effectivity only for relatively short periods of time and require frequent repetition.
During recent years there have been attempts to incorporate antifogging agents into the compositions from which agricultural plastic sheetings are produced. Such additives are generally of such structure that they have an olefinic part which is compatible with the olefinic matrix, while the other end of the molecule is of hydrophilic or polar nature, incompatible with the matrix, which is thus caused to protrude at the surface of the sheeting. When water condenses on such sheetings, it will form, through hydrogen bonding, a water film which is spread over the surface and the water will flow to the ground. A variety of such additives is known, but most of these retain their activity over not more than two or three seasons, depending on the climate and on the other components of the film. It is not possible to produce thin-gauge films containing such additives, as they tend to "slip-out" during manufacture. It is one of the objects of the invention to provide films or sheetings which retain their properties over a prolonged period of time, of the order of up to four years. There are known films having a thin EVOH (ethylene-vinyl-alcohol) layer, but this is sandwiched between two layers of polyethylene or the like, i.e. as bonding agent. Fogging is especially disturbing on the inner surfaces of polymeric sheets covering greenhouses or other agricultural structures. The droplets of water hinder the entrance of light into the greenhouse, and may reduce such entry by up to about 50 percent.
When fogging is prevented, and any water deposit forms a continuous film, this phenomenon is prevented, and there are obtained better and healthier crops. In the early morning hours the surface of the polymer sheets is the coldest surface in the greenhouse, and thus water deposits on such surfaces. This occurs generally up to about 10 o'clock in the morning. Droplets which form at the outside are generally not harmful as they evaporate rather quickly.
Experiments have shown that when fog deposition on such polymeric surfaces at the interior of agricultural structures is prevented, enhanced crops of flowers, vegetables and fruit are obtained.